1. Field of Invention
The inventions disclosed in this document relate to waveplates. More specifically, to the material/construction of waveplates used in LCoS based, quad type kernels. Kernels are the heart of light engines such as those used for video projection.
2. Discussion of Background
In the context of LCoS based kernels such as those used in light engines for video projection, waveplates are used is several different ways. For example, skew ray compensating the light rays from PBSs and residual retardation compensation in high voltage state microdisplays. Their use can be illustrated with respect to the quad style kernel 100 illustrated in FIG. #1. White light 101 is split into Red (R), Green (G), and Blue (B) light beams that are directed to corresponding colored microdisplays with the correspondingly colored video information (e.g., red light beams, directed to red microdisplays that contain red video information, green to green, etc.) in the kernel 100. Light beams reflected off the microdisplays are then combined to produce a full image. In this example, the (R) and (B) reflected light beams are combined to form the Red & Blue Image, and then the (G) image, (G) reflected light beam, is further combined with the Red & Blue Image to produce the Full Image (RGB). Waveplates (e.g., ¼ waveplates 110, 112, and 114) are inserted at each microdisplay to perform skew ray compensation (more perfectly linearly polarizing light emitted from the PBS components) and residual retardation compensation.
A half waveplate 105 is used to rotate a linear polarized image 107 containing green light by 90°. Other waveplates may be utilized in this and other kernel and/or prism assembly designs.
In current generation kernels, and other devices using waveplates, waveplates are typically made from a plastic material such as polycarbonate. The use of any plastic material presents several practical problems.
For example, some plastic waveplates are physically too thin. This makes it difficult to handle the material during the kernel assembly process. Some plastic waveplates are physically too thick. Quite often, such material is not adequately flat thus introducing phase distortion.
Available plastic waveplates of the appropriate physical dimensions and quality are of limited supply and therefore economically not suitable.
Note that the quarter waveplates (e.g., 110, 112, and 114) are used in positions close to a corresponding microdisplay (near a focal plane of the kernel). Any material near the focal plane needs to be free of included and surface defects. In an application of the kernel 100 in a projection TV, an image on the microdisplay is magnified many times (˜50x+), and even microscopic inclusions or other defects in any of the waveplates 110, 112 and 114 are readily visible in the full image. Plastic waveplates of this quality are not readily available or, again, are very expensive. Also note that all the problems and difficulties discussed herein can occur on applications other than televisions or projection devices.
Plastic waveplates are typically formed by a stretching process. Utilizing this means, it is difficult to produce waveplates with a specific retardation value. It is also difficult to produce a waveplate in which the retardation is uniform across the entire area of the waveplate.
One potential means to address these problems is to use a birefringent crystal as the waveplate. Possibly, the best candidate material is quartz.
The process of making a waveplate generally comprises making a thickness of a retarder material with a proper optical pathlength. For quartz, this requires orienting a piece of quartz with its crystal axis lined up in the correct direction and cutting the quartz to the correct thickness. However, a quarter wavelength piece of quartz has a thickness on the order of a few thousandths of an inch thick, which is generally unsuitable for many forms of mass production of items using the quarter waveplates. Therefore, quartz is suitable in many regards, but the thickness of a quarter or half waveplate is too thin for practical handling.
Wavelength specific retarders used in the industry today are typically constructed of layers of 1st order retarder components. For example, waveplates of various values are representative of the basic components of known wavelength specific retarders. The retarder components are made from materials such as polycarbonate or plastic.